Terrestrial and Jovian:

The Two Types of Major Planetary Bodies 

(Hyperlinks take you to images and other information in Bill Arnett's justly-famous "Nine 8 Planets" website.)

The major planets fall in two distinctly different categories based on their intrinsic characteristics as well as on their locations in the Solar System.  The two classes are called terrestrial ("Earthlike") and jovian ("Jupiterlike") after the largest member of each class.


Earth's Moon
(While technically a satellite and not a planet, our Moon is much like the other four terrestrials than it is like other big moons in the Solar System in terms of composition and structure.)


Pluto is very different from the terrestrials and the jovians in terms of its composition (mostly ices of lightweight materials like water) and size (it is much smaller even than Earth's Moon.)  It is not a member of either family, but rather is considered to be a "Kuiper Belt" object.

Terrestrials and jovians differ from each other in a number of ways.  Some of the most significant are:

1)  Diameter
The jovian planets are much larger than the terrestrials: the smallest jovian planet is nearly four times larger in diameter than Earth, the largest of the terrestrials.

2)  Mass
The jovian planets are all much more massive than the terrestrials: the least massive jovian contains 15 times as much mass as the Earth (and Earth is the most massive of the terrestrials.)

3)  Density
All of the terrestrial planets are rocky/metallic worlds with an aggregate average density about five times that of liquid water (so that a pint jar of typical terrestrial stuff would weigh about five pounds.)  The jovians, although large and massive, are rather "puffy": they all have very low densities, about equal to that of liquid water.  This property tells us that jovian planets cannot be primarily solid -- instead, most of their mass is in the form of liquid hydrogen.

4)  Chemical Composition
The jovians seem to have about the same chemical composition (by element) as the Sun: 75% hydrogen, 24% helium, and only 1% everything else.  The terrestrials, however, being unable to hold on to the lightest gases gravitationally, have very little hydrogen and helium, and are made mostly of much heavier elements.  The most abundant atom in the body of the Earth, for example, is oxygen (mostly locked up in molecules with other elements), each atom of which weighs 16 times as much as each hydrogen atom does.

5)  Magnetic Field
The terrestrials have very much weaker magnetic fields than the jovians do.  Terrestrials' fields are generated in their small metal cores; jovians' fields are generated in much larger regions of conducting fluids ("metallic" hydrogen in the super-pressurized cases of Jupiter's and Saturn's interiors.)

6)  Number of Natural Satellites (Moons)
The terrestrial planets are almost devoid of natural satellites: Mercury and Venus have none, Earth only one, and Mars only two very tiny ones -- and those latter two are probably captured asteroids.  Jovian planets have a large number of satellites each, and exactly how many they have depends on how small an object one is willing to call a "moon."  At least six of the satellites in the outer solar system are larger than Pluto, and one is bigger than Mercury.

7)  Rotation Rates
The jovian planets rotate (spin) more rapidly than do the terrestrials.  The slowest-rotating jovian planet, Uranus, has a rotation period that's less than 18 hours long; Earth's 24-hour period is the shortest among the terrestrials (and Venus takes 243 times longer than that for one full spin!)  Jupiter is so big and spinning so fast that its equator is moving about 25,000 miles per hour relative to the center of the planet: this, combined with Jupiter's primarily liquid interior, makes Jupiter noticeably oblate (flattened), with a polar diameter that's only about 90% as big as its equatorial diameter.

8)  Distance from the Sun
All of the jovian planets are farther from the Sun than the terrestrials are.  This probably accounts for the differences in chemical composition: the inner planets had their original hydrogen and helium atmospheres "blasted away," perhaps, by the early Sun's heat.  The differences in chemical composition, in turn, may account for the differences in mass and diameter (since hydrogen and helium are the most abundant elements in the universe) and in density.

This is a page of notes posted for a DeAnza College introductory astronomy class.  Any questions or comments should be sent to its author, Sherwood Harrington.
Last updated: November, 2009